1. Field of the Invention
The present invention relates to a process for removing metals such as sodium, iron, calcium, and the like contained in homopolymers of vinylphenols, copolymers of vinylphenols and other comonomers or modified products of these, or derivatives of these (hereinafter these polymers are collectively called "vinylphenol-type polymers"), by passing each of these polymers which are in a solution form, through a specific filter.
2. Description of the Prior Art
Vinylphenol-type polymers are used in the field of electronics such as photoresists, packaging materials for IC, printed-circuit boards, and the like. In order to use these vinylphenol-type polymers as materials for devices, and the like for which the precision have been more improved in recent years, it is required that the content of metal impurities in these materials be extremely low. It is thought that the metal content, particularly, of photoresists used for the precise processing of LSI of 256 megabits or more must be reduced to an extremely low trace content in the order of ppb units.
Various processes are known for manufacturing vinylphenol-type polymers. A process in which a monomer of vinylphenol produced by various processes, for example, dehydration of acetoxyphenylmethyl carbinol, decarboxylation decomposition of hydroxycinnamic acid, dehydrogenation of ethylphenol, or the like is homopolymerized or copolymerized with other comonomers in the presence of a cationic catalyst or a radical initiator is known as an industrial process.
The following process is also known. Specifically, a monomer in which a hydroxyl group of vinylphenol is protected by an acetyl group, trimethylsilyl group, t-butyl group, t-butoxycarbonyl group, or the like is synthesized. This monomer is polymerized in the presence of a cationic or anionic catalyst, or radical initiator to prepare a polymer, followed by removing the capped group. In this process, a monodisperse polymer can be produced by a so-called living polymerization depending on the selection of the protective group and the initiator (Japanese Patent Application Laid-open No. Hei 3(1991)-277608).
Further, there are known processes including a process in which each of these vinylphenol-type polymers is converted into a modified product having excellent light transmittance by hydrogenation (Japanese Patent Application Laid-open No. Hei 1(1989)-103604), a process of manufacturing various ether or ester derivatives by the reaction of hydroxyl groups of these vinylphenol-type polymers (Japanese Patent Application Laid-open No. Sho 63(1988)-312307 and Fiber and Polymer Material Research Institute Report, Vol. 128, page 65 (1981)), and a process of manufacturing various kinds of nuclear substituted products of vinylphenol-type polymers (Japanese Patent Application Laid-open No. Sho 51(1976)-83691).
However, in the vinylphenol-type polymers produced by or modified by these processes, contamination by metals occurs, for example, by sodium, iron, calcium, and the like which are contained in the starting raw materials or auxiliary materials or derived from materials of manufacturing facilities used in manufacturing processes and from contamination of facilities and environments.
For this reason the following processes have been proposed in the past:
(a) A process in which a vinylphenol-type polymer dissolved in a solvent is allowed to contact with a strongly acidic cation exchange resin (U.S. Pat. No. 5,288,850); PA1 (b) a process in which a vinylphenol-type polymer dissolved in a solvent is treated by a specific hydrogenating process, then the solution treated in the hydrogenating process is subjected-to a metal removing process in which the solution is allowed to contact with a strongly acidic cation exchange resin (U.S. Pat. No. 5,284,930); PA1 (c) a process in which a vinylphenol-type polymer dissolved in an organic solvent is allowed to contact with a solution containing an acidic compound and then contact with an ion exchanged water (Japanese Patent Application Laid-open No. Hei 6(1994)-192318); and PA1 (d) a process in which a vinylphenol-type polymer dissolved in a solvent is allowed to pass through a filter generating a zeta (.zeta.) potential, i.e. interfacial electrokinetic potential, by a cationic charge modifying agent (Japanese Patent Application Laid-open No. Hei 8(1996)-165313).
There are, however, the following problems in these processes (a) and (b) contacting with a strongly acidic cation exchange resin. Specifically, the use of a packed column with a length of several tens of centimeters is usually required in the case of using these processes in a practical, industrial scale, and the flow rate of the solution of vinylphenol-type polymer cannot be maintained so high, in order to keep a high degree of metal removal, resulting in poor productivity. In fact, the references state that liquid hourly space velocity (LHSV) of the solution to be treated is 0.2-5 h.sup.-1, and LHSV of 0.2 h.sup.-1 is used in Example 1 of both references. Also, because the residence time in the column is long, and the like, if an inappropriate solvent is used, the solvent is partly decomposed to produce an acid, and the acid is mixed in the solution so that there is the case where a vinylphenol-type polymer thus obtained is not suitable for photoresist materials.
The process (c) contacting with an aqueous solution containing an acidic compound has the following drawbacks. Specifically, if a water-soluble solvent with a low boiling point such as methanol, acetone, or the like is used, vinylphenol-type polymer is precipitated in the case of the process (c), notwithstanding the solvent usually well dissolves vinylphenol-type polymer. Also, when a solvent which has a high boiling point and has a tendency to easily separate from an aqueous solution in general is used, there is the case where the vinylphenol-type polymer solution and the aqueous solution form an emulsion to cause difficulty in separation whereby the kind of usable solvent is limited. Also, distillation under a reduced pressure is required to remove water from the solution after washing, and thus complicating the process.
The process (d) removing metals by a filter generating a zeta (.zeta.) potential is an attractive process because a considerably high degree of metal removal can be attained by only simple operation such as filtration and the solution can be treated at a high flow rate, leading to high productivity. However, the total amount of metals which can be trapped is comparatively small and the life of the capability of the filter of removing metals is short. It is necessary that the filter be replaced in a short period of time and hence process (d) is economically unsatisfactory.